The present invention generally relates to thermal barrier coating systems for components exposed to high temperatures, such as airfoil components of gas turbine engines. More particularly, this invention is directed to a thermal barrier coating system and process for selectively depositing multiple ceramic layers on different surface regions of a component to reduce surface temperatures and temperature gradients within the component.
Components within the hot gas path of a gas turbine engine are often protected by a thermal barrier coating (TBC) system. TBC systems include a thermal-insulating topcoat, also referred to as the thermal barrier coating or TBC. Ceramic materials are used as TBC materials because of their high temperature capability and low thermal conductivity. The most common TBC material is zirconia (ZrO2) partially or fully stabilized by yttria (Y2O3), magnesia (MgO) or another alkaline-earth metal oxide, ceria (CeO2) or another rare-earth metal oxide, or mixtures of these oxides. Binary yttria-stabilized zirconia (YSZ) has particularly found wide use as the TBC material on gas turbine engine components because of its low thermal conductivity, high temperature capability including desirable thermal cycle fatigue properties, and relative ease of deposition by thermal spraying (e.g., air plasma spraying (APS) and high-velocity oxygen flame (HVOF) spraying) and physical vapor deposition (PVD) techniques such as electron beam physical vapor deposition (EBPVD).
To be effective, TBC's must remain adherent through many heating and cooling cycles. This requirement is particularly demanding due to the different coefficients of thermal expansion between ceramic materials and the superalloys typically used to form turbine engine components. As is known in the art, the spallation resistance of a TBC can be significantly improved with the use of an environmentally-protective metallic bond coat. Bond coat materials widely used in TBC systems include overlay coatings such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth or reactive element such as hafnium, zirconium, etc.), and diffusion coatings such as diffusion aluminides. When subjected to an oxidizing environment, these aluminum-rich bond coats develop an aluminum oxide (alumina) scale that is advantageously capable of chemically bonding a ceramic TBC to the bond coat and the underlying substrate.
Spallation resistance is also influenced by the TBC microstructure, with greater spallation resistance generally being achieved with microstructures that exhibit enhanced strain tolerance as a result of the presence of porosity, vertical microcracks, and/or segmentation. As used here, the term “segmentation” refers to a TBC with columnar grains oriented perpendicular to the surface of the component, such as that achieved with PVD processes such as EBPVD. The term “vertical microcracks” refers to fine cracks that are intentionally developed in thermal sprayed TBC's, whose microstructures otherwise generally consist of “splats” of irregular flat (noncolumnar) grains formed by solidification of molten particles of the TBC material. Plasma-sprayed TBC's with microcracks are discussed in U.S. Pat. Nos. 5,073,433, 5,520,516, 5,830,586, 5,897,921, 5,989,343 and 6,047,539. As is known in the art, ceramic TBC's having columnar grains and vertical microcracks are more readily able to expand with the underlying substrate without causing damaging stresses that lead to spallation.
The demand for higher temperatures to improve efficiency and reduce emissions puts additional demands on gas turbine engine components within the hot gas path. For example, the blade tips and inner platforms of high pressure turbine (HPT) blades and vanes are subjected to significantly higher temperatures within engines equipped with combustors having relative flat profiles to reduce emissions. Several methods are available for effectively cooling the airfoil and tip of a turbine blade, such as with bleed air that flows through internal passages within the blade and exits cooling holes on the surface of the airfoil and/or blade tip. Attempts to air cool blade platforms are complicated by the desire to avoid internal and surface features that could increase stress concentrations which, in combination with thermal gradients typically within platforms, can lead to cracking. Additionally, there can be regions of a platform that have low back flow margin. Though blade platforms generally see lower temperatures than blade tips, the thermal gradient within a platform can result in platform cracking if the airfoil is effectively cooled but the platform is not.
In view of the above, it would be desirable if a relatively thick TBC could be deposited on blade platforms to provide additional thermal protection and reduce the thermal gradient through the platform thickness. The process most often used to deposit TBC on air-cooled turbine blades is the above-noted EBPVD technique due to its ability to apply a thin, uniform coating without plugging the small cooling holes in the airfoil surface. However, TBC thicknesses capable of adequately reducing the surface temperature of a platform risk plugging the airfoil cooling holes. While the relative amount of TBC deposited on the platform can be increased by tilting the blade relative to the vapor source, the limitations of existing EBPVD equipment are such that a sufficiently thick TBC cannot be deposited on the platform without also depositing an excessively thick TBC on the airfoil. Another problem is that the erosion resistance of EBPVD TBC decreases to some degree if the surface being coated is other than parallel to the surface of the vapor source. As such, tilting a blade to increase the relative amount of TBC deposited on the platform can unacceptably reduce the erosion resistance of the TBC on the airfoil. Finally, the deposition rate on an inclined surface is relatively lower, thus increasing the time and cost of the deposition process.